The present invention relates to a particular type of the Inorganic Light Emitting Devices based on zinc sulfide.
ZnS is a well-known phosphor material. It is used in many applications like after-glow phosphors, photon conversion phosphors and electroluminescent phosphors (Cathode Ray Tube displays, Field Emission Displays, Powder Electroluminescent devices, . . . ).
Back in 1928, Lenard(1) and his group found that incorporation of certain metal impurities such as Cu, Ag or Pb leads to luminescence characteristics of these ions. For this reason he called such an impurity an xe2x80x9cactivatorxe2x80x9d. He also found that ZnS without these impurities can show blue luminescence if fired with alkali halide flux. This was called xe2x80x9cself-activatedxe2x80x9d luminescence. It was later clarified that self-activated luminescence is actually activated by pairs of a halogen donor and a Zn-vacancy acceptor.
Clear explanation of the luminescence centers and the mechanism of optical transitions were established in 1960s(2).
In parallel with scientific advances, some discoveries and inventions motivated research of ZnS phosphors aimed for applications. Destriau(3) discovered electroluminescence from ZnS powder immersed in oil and sandwiched between metal plates, when applying electric voltage to the metal plates. In the early 1950s Sylvania announced a flat lamp using this phenomenon. Today, commercial ZnS-based phosphor powders formulated in pastes are available. These pastes, together with a suitable dielectric paste and conductor pastes, can be used in simple screen printing processes for producing flat lamps with almost unlimited geometric forms.
Later, Sharp cooperation(4) developed driven thin-film El panels, in which ZnS:Mn2+ is sandwiched by insulator films. Since 1983 orange-emitting monochrome displays using ZnS:Mn2+ films have been commercialized.
In color Cathode Ray Tube displays, first Zn2SiO4:Mn2+ for green primary and Zn3 (PO4)2:Mn2+ for red were used with blue-emitting ZnS:Ag(5). Later the xe2x80x9call-sulfide screenxe2x80x9d using ZnS-based materials for the three primaries were put on the market. Although in 1964 the red-emitting YVO4:Eu3+ was developed and later in 1968 Y2O2S:Eu3+, which is the red phosphor presently used(6), ZnS phosphors have been always very important green and blue phosphors.
Semiconductor particles of several nm size show various interesting phenomena originated in the transitional nature from molecular to bulk properties. When the particle size becomes smaller than the diameter of a bulk exciton (e.g. about 6 nm in CdS and ca 4.4 nm in ZnS(7), the optical spectra will show size-dependent absorption thresholds instead of the constant bulk band gap. With decreasing particle size, the threshold shifts to higher photon energy. Such phenomena are called xe2x80x9cquantum size effectsxe2x80x9d or xe2x80x9cquantumconfinementxe2x80x9d.
Some examples of wet chemical preparations of ZnS nano-particles can be found in the following references(8).
Becker(9) observed a broad photoluminescence peak of colloidal ZnS around 428 nm which he assigned to transitions due to sulfur vacancies. After irradiation treatment, the peak shifted to 418 nm and this was attributed to transitions involving interstitial zinc or sulfur atoms. D. Denzler et al.(10) describes the photoluminescence behaviour of colloidal ZnS nanocrystals. In the UV-region a quadruple fine structure could be observed at wavelengths: 416, 424, 430 and 438 nm. In the visible region a weak band at 590 nm could be observed. However, this was attributed to Mn2+ impurities. In the IR-region, two fine structured emission bands at 675 and 715 could be observed.
In pending patent applications different thin film electroluminescent devices and constructions are described. Apart from the well-described organic based emitting devices like PLEDs and OLEDs and the inorganic based emitting devices like PEL and TFEL, several research groups reported recently electroluminescence(11-15) from inorganic semiconducting nano particles.
Colvin et al.(11) reported on the electroluminescence of CdSe nano-particles stabilized by hexane dithiol. They demonstrated electroluminescence for two devices comprising a spincoated double layer of CdSe and PPV on ITO and covered it with an evaporated Mg electrode. Depending on the voltage they observed emission from the CdSe (lower voltages) or from the PPV (higher voltages).
Electroluminescence of CdSe quantum-dot/polymer composites was also reported by Dabbousi et al.(12). They spincoated on ITO one single layer of CdSe nano-particles stabilized with trioctylphosphine oxide and mixed with a polymeric hole transporter (PVK) and an electrontransport species (an oxadiazole derivative of PVK, t-Bu-PBD). An aluminum electrode was subsequently evaporated. The system showed electroluminescence in reverse bias, and depending on the applied voltage the emission spectrum of the CdSe quantumdots or PVK was observed.
Gao et al.(13) reported on the electroluminescence of self-assembled films of PPV and CdSe nano-particles. They could observe electroluminescence from the CdSe particles and/or from the PPV, depending on the applied voltage.
These examples demonstrate the possible use of inorganic nano-particles with semiconductor properties as Light Emitting Diodes (ILED), in analogy with the OLEDs. However, the use of Cd- or Se-compounds can not be recommended due to environmental problems that can be expected.
Huang et al.(14) reported the photo- and electroluminescence of a single layer of ZnS:Cu nanocrystals spincoated on a ITO substrate and evaporated with an aluminum electrode. ZnS and CuxS are much more environmental friendly compared to CdSe. Also there was no need for organic hole or electron transporters, which can cause stability problems as is known in the organic PELDs. The drawback of their system lies in the fact that the synthesis of the ZnS:Cu particles is quite cumbersome and results in low yields. Polystyrene sulphonic acid is used as polyelectrolyte on which Zn and Cu ions are attached. Subsequently this polyelectrolyte is solved in dimethylformamide and reacted with H2S. By this way ZnS:CxS particles are formed.
Que et al.(15) reported photo- and electroluminescence from a copper doped ZnS nanocrystals/polymer composite. The synthesis of the nano-particles was carried out by using the inverse microemulsion method. After washing and drying the ZnS:Cu powder was redispersed in methylethylketone (MEK) with polymethylmethacrylate (PMMA) as a binder and spincoated on ITO and evaporated with an aluminum electrode. Green electroluminescence could be observed in both bias directions at 5 V. The drawback of the fabrication of this device is the low concentrations of the ZnS:Cu dispersion that can be obtained (ca 10xe2x88x923 M). Further it needs a well defined two phase system (soap/water). Also a drawback could be the solvent based spincoating dispersion.
Leeb et al.(16) describes the electroluminescence of ZnS:Mn nano-particles. The device could be made more stable by adding ZnI2.
In pending patent applications(8e) ILEDS are described which make use of doped ZnS nanoparticles.
All ILED examples mentioned above use photoluminescent nano-particles: or they are photoluminescent due to their quantumconfinement (like CdS, CdSe) or they are photoluminescent due to their doping (like ZnS:Mn2+, ZnS:Cu+, ZnS:Cu2+). In both cases, however their emission bands are quite broad, all photoluminescence and hence all electroluminescence results in colored emission, i.e. no white emission is observed. The only way to produce white electroluminescent light is to combine different emitting particles or to use luminescent dyes in order to broaden the emission band.
A similar strategy was developed for PLEDs: mixing of different emitting polymers can result in white emitting PLEDs. In full-color displays, the summation of the three emission bands of the three basic colors, which are pixelated in this case, creates a white pixel, i.e. white emission.
However, due to the differences found in the operating lifetimes of the emitting polymers, the white color will not be stable due to the faster decay of one of the emitting polymers: the emitted light will become colored. This is also the main reason why full-color OLED or PLED displays are not yet commercial available.
A stable white emitting thin film device could be used as backlight in a full-color LCD device. If pixelated, the use of a color filter, also used in emitting LCD applications, will allow to construct a very simple straigthforeward full-color display without the drawbacks mentioned above.
References Cited:
(1) P. Lenard, F. Schmidt and R. Tomaschek, Handbuch der Experimantalphysik, Akademische Verlagsgesellschaft, Leipzig, 1928, p. 397.
(2) S. Shionoya In: xe2x80x9cLuminescence of Inorganic Solidsxe2x80x9d, (Ed. P. Goldberg, Academic Press), 1966, pp. 205
(3) G. Destriau, J. Chim. Phys. (France), 1936, 33, 587
(4) T. Inoguchi, M. Takeda, Y. Kakihara, Y. Nakata and M. Yoshida, SID""74, 1974, Digest 84.
(5) H. Yamamoto, Lumin. Relat. Prop. II-VI Semicond., 1998, 169-207, ed. Vij D. R. Singh N., Publisher: Nova Science Publ. Zinc Sulphide.
(6) M. R. Royce and A. L. Smith, 1968, rare earth Oxysulfidesxe2x80x94A new family of phosphor hosts for rare earth activators, Electrochem. Soc., spring Meeting, Abstract No. 34.
(7) K. H. Hellwege, O. Madelung, M. Schult and H. Weiss, Landolt-Borstein, Semiconductors (Springer, Berlin, 1983), Vol. 17
(8) (a) R. Rossetti, R. Hull, J. M. Gibson and L. E. Brus, J. Chem. Phys., 1985, 82, 552; (b) H. Weller, U. Koch, M. Gutixc3xa9rrez and A. Henglein, Ber. Bunsenges. Phys. Chem. 1984, 88, 649-656; (c) J. G. Brennan, T. Siegrist, P. J. Carroll, S. Stuczynski, P. Reynders, L. E. Brus and M. L. Steigerwald, Chem. Mater. 1990, 2, 403;(d) R. N. Bhargava, D. Gallagher, X. Hong and A. Nurmikko, Phys. Rev. Lett., 1994, 72, 416; (e) European patent applications, Appl. Nos. 01000005, 01000006, 01000007, 01000008, 01000009, 01000010.
(9) W. G. Becker and A. J. Bard, J. Phys. Chem. 1983, 87, 4888
(10) D. Denzler, M. Olschewski and K. Sattler, J. Appl. Phys., 1998, 84, 2841-2845
(11) Colvin V. L., Schlamp M. C. and Alivisatos A. P., Nature, 1994, 370, 354-357.
(12) Dabbousi B. O., Bawendi M. G., Onitska O. and Rubner M. F., Appl. Phys. Lett. 1995, 66 (11), 1316-1318
(13) Gao M., Richter B., Kirstein S. and Mxc3x6hwald H., J. Phys. Chem. B 1998, 102, 4096-4103
(14) Huang J., Yang Y., Xue S., Yang B., Liu S., Shen J. Appl. Phys. Lett. 1997, 70(18), 2335-2337
(15) Que, Wenxiu; Zhou, Y.; Lam, Y. L.; Chan, Y. C.; Kam, C. H.; Liu, B.; Gan, L. M.; Chew, C. H.; Xu, G. Q.; Chua, S. J.; Xu, S. J.; Mendis, F. V. C.; Appl. Phys. Lett. 1998, 73(19), 2727-2729.
(16) J. Leeb, V. Gebhardt, G. Mueller, D. Haarer, D. Su, M. Giersig; G. McMahon, L. Spanhel, J. Phys. Chem. B, 1999, 103(37), 7839-7845.
It is an object of the present invention to provide a Thin Layer Inorganic Light Emitting Device based on zinc sulfide, which is easy to manufacture.
It is a further object of the invention to provide a Thin Layer Inorganic Light Emitting Device based on zinc sulfide which is capable of emitting light with an emission wavelength above 450 nm, preferably stable white light.
It is still a further object of the present invention to provide a Thin Layer Inorganic Light Emitting Device based on zinc sulfide which can be used as backlight in a full-color LCD device.
The objects of the present invention are realized by providing a Thin Layer Inorganic Light Emitting Device comprising, in order,
a transparent or semi-transparent substrate,
a first electrode,
a coated layer comprising zinc sulfide nanoparticles,
a second electrode, with the proviso that at least one of said first and second electrodes is semi-transparent,
characterized in, that (a) said zinc sulfide nanoparticles substantially contain no metal impurities, and (b) the device is capable of emitting light in response to a direct current caused by applying a voltage between said first and second electrode, with an emission maximum of electroluminescence at a wavelength above 450 nm.
In a preferred embodiment the emission maximum is in the visible spectral region.